It’s easy to assume that the nation’s richest counties dot the tri-state area around New York City, or San Francisco’s Bay Area. Homeowners there, after all, shell out millions of dollars for small luxuries such as a lawn and a garage.

But while affluent areas such as Nassau County, on New York’s Long Island, and Marin County, just north of San Francisco, boast well-off residents, the nation’s wealthiest live in the D.C. suburbs.

Fairfax County, Va., Loudoun County, Va., and Howard County, Md., top the list of America’s richest counties, which we based on median household income data from the 2006 census. In Fairfax, that number reaches $100,318 a year; Loudoun households pull down a livable $99,371 a year; Howard residents follow at $92,260.

Yet D.C. is not generally regarded as a center of wealth. What about the billions handed out in bonuses to Wall Street execs, or the seemingly never-ending stream of Silicon Valley-based tech billionaires?

Those factors have an impact on the ranking, helping drive New Jersey’s Hunterdon County and California’s Santa Clara County, near San Jose, to Nos. 4 and 17, respectively. But we are not measuring what the extremes earn, only the middle. Except for Santa Clara, none of the richest counties we found are in cities that contain some of the country’s most expensive homes, which only a select few can afford. Instead, suburbs are where most of the country’s money goes to live.

“The general process of suburbanization is, the richer you are, the more likely you move to the suburbs,” says Julie Martin, a senior demographer for the Weldon Cooper Center for Public Service at the University of Virginia.

In the case of Washington, D.C., she says that well-paid government employees, and the area’s lobbyists, lawyers and other tangential personnel, are “suburbanizing like mad” and in doing so have created a “feedback loop,” driving more money into the area’s suburbs by populating them with higher-priced homes and better school systems.

This often results is significant income differentiation among neighboring counties. In Forsyth County, Ga., for example, the median household earns $83,682 a year, about $33,000 more than Fulton County, its immediate neighbor. The more a county becomes an established bastion of wealth, the more residents from the surrounding areas want to move there, driving up prices for homes and creating superior tax-based services.

Income Issues
Money creates its own set of problems, however. The most straightforward example: High home prices, and the subsequent boon to tax collectors, mean money for schools but often a paucity of teachers, as educators often cannot afford high home prices.

While an influx of high-earners might fortify an area, money managers and corporate lawyers still need people to teach their kids, and firefighters and police officers to protect them. To make sure these civil servants can live where they work, counties filled with posh suburbs need to provide affordable housing for those who couldn’t otherwise live there.

Consider Douglas County, Colo., just outside Denver and No. 5 on our list of richest counties. It offers below-market interest rates as well as interest-free, five-year, payment-deferred loans of $20,000 to teachers, firemen, and policemen. The county also incentivizes builders to offer those buyers $5,000 discounts on home prices, or to waive closing costs.

Sounds fair, and it might even have a silver lining in this market.

“The home sales market isn’t what it was,” says Wendy Holmes, a Douglas County spokeswoman, on the housing downturn. “But these incentives are helping to move homes, especially for workers that service everybody.”

© 2008 Forbes.com LLC™